You can use either method mentioned above to inlet the rear entry pipe. I usually inlet one of the forward tailless pipes first, then do the rear pipe tail afterwards. Know this too - when it is time to drill for the pins you will want to
gently clamp a pipe to the stock before drilling, this ensures the pipe is fully down, and snug, in the ramrod channel and your pins will hold the pipe firmly in place.
I use a finger drill for the actual drilling, note again, there is only enough clamping force to hold the pipe firmly in place, no more. Without the clamp the pipe may wiggle once you've installed the pins, this is especially annoying on a rear entry pipe.
Go slow but sure...
dave